“In Russia it is still [an] old-fashioned and old-style KGB system... It's still all the same. If there is an order to kill somebody it will happen.”

In an interview with the Telegraph, she added: “It looks similar to what happened to my husband but we need more information.

“We need to know the substance. Was it radioactive?”

But in a statement today, the Kremlin claimed “we don't have any information” about the “tragic situation” involving Mr Skripal.

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KREMLIN TARGET: Former KGB agent Alexander Litvinenko was poisoned in London in 2006

"I cannot voice any response because we don’t have any information," Kremlin spokesman Dmitry Peskov said.

Moscow has not been asked for help by UK authorities but “is always open to cooperation", he said.

Asked about the link being made in the media between Mr Skripal and the death of Mr Litvinenko, Mr Peskov said: "It didn't take them long."

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DOUBLE AGENT: Sergei Skripal was convicted of treason for passing state secrets to MI6

Mr Rowley told BBC Radio 4's Today programme: "We are doing all the things you would expect us to do. We are speaking to witnesses, we are taking forensic samples at the scene, we are doing toxicology work.

"That will help us get to an answer, I can't say any more at this stage."

Asked about a series of suspicious Russian-linked deaths in the UK, Mr Rowley said: "There are deaths which attract attention.

"I think we have to remember that Russian exiles are not immortal, they do all die and there can be a tendency for some conspiracy theories.

"But likewise we have to be alive to the fact of state threats as illustrated by the Litvinenko case."